High energy density radiation beam welding, such as is accomplished using a laser beam or electron beam, is becoming more widely used to join one sheet to another sheet during the course of manufacturing a product made up of at least in part by the sheets. Laser welders use a highly focused beam of light energy directed onto one or both sheets to join them together, while electron beam welders direct a stream of electrons onto the sheets to be welded to heat them and fuse the sheets together.
One type of welding joint commonly used to secure one sheet to another sheet is a lap joint where an edge portion of one sheet is placed in overlapping fashion against an edge portion of another sheet before welding the sheets together in the overlap region. Typically, before welding, the sheets are held against each other by clamps and the beam is directed against one or both of the sheets to join them together in the overlap region.
In one type of lap joint weld, a partially or fully-through penetrating weld is used to join the sheets together in what is referred a lap seam weld. In producing a lap seam weld, the beam is directed against the upper surface of the top sheet with enough energy density and for a sufficient period of time such that the beam melts and fuses through the top sheet penetrating completely through the top sheet and at least partially through the bottom sheet. However, this type of lap joint welding method produces a weld seam which does not cover the entire overlapping surfaces leaving at least some portion of the overlapped surfaces unwelded.
This results in a lap joint that is open and, therefore, not completely fused where the sheets overlap each other, making it possible, even highly likely, for the sheets to peel apart from each other, as well as wrinkle or buckle, in the region of the lap weld during post-welding forming of the sheets or for the lap seam weld to fail during use of a finished product constructed of the sheets. In addition to low peel strength, an open lap joint can have unfused areas where cracks can easily form, initiating undesirable failure of the weld joint after repetitive or cyclical loading such as what can occur during use of a finished product constructed of the sheets. These unfused areas of the lap weld joint can also collect moisture and contaminants leading to undesirable corrosion in the weld joint which can later also lead to failure of the open lap weld. Additionally, the shear strength of the weld may be poor if the weld is not wide enough which can contribute to poor fatigue strength that, in turn, can cause premature weld failure.
Unfortunately, the integrity of the sheets can be negatively impacted in other ways if the weld is too wide, such as can be characteristic of the laser-mash seam lap welding method disclosed in Budendbender, U.S. Pat. No. 4,945,202. For example, if sheets are coated with a corrosion resistant coating, such as a zinc coating, heat generated during welding can vaporize the coating leaving the sheets unprotected in the area of the weld. If this unprotected area is too large, the "self-healing" properties of the zinc coating may not be able to prevent corrosion from forming in the weld area as well as in the heat affected zone (HAZ) surrounding the weld. Moreover, even if a lap seam weld of sufficient width could be produced so that it joined the sheets together completely across the overlap of the sheets by weaving the beam across the overlap with the beam impinging on the exposed top surface of the top sheet, it would require a relatively large amount of energy and adversely affect the formability of the sheets because the weld would have a rather large cross-sectional area and surrounding heat affected zone resulting in a more brittle weld region that would not lend itself to be formed successfully, such as by bending, deep drawing, roll-forming, flanging, piercing, or another forming method. Moreover, weaving the beam across the overlap reduces the weld speed and destroys coating on the beam impinging surface, that is, the top surface of the top sheet, and induces coating contamination in the weld nugget.
In addition to the lap seam joints just discussed, another type of lap joint is a lap fillet joint. In a lap edge joint, the sheets are overlapped such that their edges are parallel and generally in line with each other. Unfortunately, for all of these types of lap joints, the joint is open with portions of the overlap unwelded, dramatically reducing peel strength and increasing the likelihood for corrosion as well as making these joints difficult to form or shape after welding without weld failure or forming defects such as wrinkling or buckling occurring.
An alternative to the aforementioned lap joint constructions is conventional mash seam welding, such as is disclosed in Kerby, U.S. Pat. No. 3,159,419. As is disclosed, a pressure roll electrode on each side of the overlapped sheets melts, fuses and welds the sheets together across the overlap region while pressure applied by the rolls reduces the thickness of the overlap. After resistance mash seam welding, the sheets form part of a blank that is formed for later assembly as part of an automobile. However, because of the relatively large cross-sectional size of these welds and its associated HAZ, the weld joints are located away from areas of the blank that are greatly formed or bent.
There are other disadvantages to resistance mash seam welding in addition to lacking formability. One further disadvantage is that resistance mash seam welding is not suited for lap joint welding of more complex two- and three-dimensionally contoured sheets. A still further disadvantage is that any low vaporizing temperature coating on the sheets, such as zinc coating, is vaporized over a wide area during welding which can leave the lap joint with little or no corrosion protection. For sheets having such a coating on both sides, the coating can also vaporize in the overlap area and become trapped in the molten metal leading to welds possessing poor integrity which can lead to premature weld failure.
Lentz, et. al, U.S. Pat. No. 4,769,522, discloses a method of laser welding using a sophisticated apparatus and fixture for joining overlapped ends of a sheet to form a container body. A laser beam is directed into a "mouth" where the sheets are overlapped and acutely angled relative to each other with the beam impinging against portions of the adjacent surfaces of the overlapped sheet ends to heat them above their melting temperature. Before solidifying, the "mouth" is closed by the fixture pressing the sheet ends into overlapping contact with each other to fuse them together in a lap joint arrangement.
The method disclosed in Lentz requires that the sheet ends be accurately spaced apart from each other, accurately located, and fixtured relative to one another during scanning of the sheet by the laser beam before quickly bringing the sheet ends into contact with each other before solidification occurs. Of course, should the sheet ends not be pressed together quickly enough before the sheet solidifies, the resulting weld joint is of poor integrity and low peel strength, both of which could lead to premature weld failure. Therefore, this welding method is susceptible to leaving portions of the overlap unfused reducing significantly formability and weld peel strength while resulting in decreased corrosion protection in the weld region.
An article in the December 1993 issue of the periodical Welding Journal entitled "Laser Beam Welding Goes into High-Speed Production of Home Hot Water Tanks", discloses a method of laser welding for joining together two generally cylindrical tank halves to form a tank of a hot water heater. The bottom tank halve has a radially outwardly turned lip for guiding the top tank halve into tight-fitting, intimate contact with the lower halve necessary for welding the two halves completely about their peripheries. There must be no fit-up gap anywhere along the weld joint interface where the upper and lower tank halves are overlapped so that the laser beam will not pass through the joint interface during welding. Unfortunately, the unwelded portion of the outwardly turned lip of the upper tank halve used to guide the lower tank halve into tight-fitting contact produces an open lap joint adversely affecting the peel strength of the weld joint. Additionally, because this welded joint is open, it is also susceptible to crack formation and corrosion. Furthermore, if deep drawing of the weld region is attempted after welding is performed, the weld can yield to crack failure because of reduced peel strength in the weld region and due to wrinkling and buckling in the area of the lap joint because of the aforementioned unwelded overlapping outwardly turned lip.
Autogenous laser and electron beam butt-welding methods have been used to form blanks prior to forming them, as is disclosed in the 1992-93 edition of The Industrial Laser Handbook entitled "Tailored Welded Blanks: A New Alternative in Automobile Body Design" and in an August 1974 article in the periodical Welding Journal entitled: "Production Electron Beam Welding of Automotive Frame Components". In constructing a tailored welded blank component, a first sheet is cut to the desired shape and butt-welded to another sheet using a laser or electron beam gun. After welding, the component is formed. However, before butt-welding, accurate edge preparation of the sheets where they are to be joined must be done so that the edge of one of the sheets is virtually perfectly parallel to the edge of the other sheet before butt-welding can be performed.
Even when done properly, however, edge preparation is time consuming and costly. Typically, during edge preparation, the edge of each sheet to be joined is machined to make it parallel with the edge of the other sheet so that there is virtually no gap between the sheets when they are butted against each other for welding. If edge preparation is not properly performed, when the edges of the sheets are butted against each other to be welded, any gap that is too large between the sheet edges can result in a poor weld joint between the sheets. This can lead to the sheets not being joined together in that area, possibly causing weld failure during forming or even more undesirably, later, when the sheets are in use.
Conventional welding methods, such as shielded metal-arc, submerged arc, self-shielded flux-core and gas-shielded arc welding processes, are also not suitable because the resultant welded sheets cannot be easily formed, especially deeply drawn, in the region of the weld without negatively impacting weld strength and weld integrity, possibly resulting in weld failure during forming as well as cracking or peeling of the weld joint as well as wrinkling or buckling in the region of the weld. Conventional welding methods are also ill-suited for welding sheets having corrosion resistant or low vaporizing temperature coatings because they produce relatively wide welds destroying the coating across the region of the weld and its surrounding HAZ. Finally, the production rates that can be achieved using conventional welding processes are relatively slow, further making their use in these types of applications economically undesirable.